Radio Frequency Identification (“RFID”) is a generic term for technologies that use radio waves to automatically identify individual items. Objects can be identified using RFID by storing a serial number that identifies the object on a chip that is attached to an antenna. The chip and the antenna together are called an RFID tag. An RFID reader sends out electromagnetic waves that are received by the antenna on the RFID tag. Passive RFID tags draw power from this electromagnetic field to power the chip. Active tags use their own batteries to power the chip. There is also a version of a passive tag that does contain a battery, and is referred to as an “active/passive” tag. This tag has some of the attributes of a true active tag, but communicates in the same manner as a passive tag.
RFID tags can also be distinguished by their memory type. Read/write memory, can be read as well as written into such that its data can be dynamically altered. Read only (typically “chipless”) type of tag memory is factory programmed and cannot be altered after the manufacturing process. According, the data of the RFID tag is static.
A typical passive RFID tag is shown by FIG. 1, and generally indicated by symbol 10. The RFID tag comprises an antenna circuit 20 and an integrated circuit 30. The integrated circuit 30 may include an RF (or AC) rectifier 40 that converts RF (or AC) voltage to DC voltage, a modulation circuit 50 that is used to transmit stored data to a tag reader, a demodulation circuit 60 that is used to receive data from the tag reader, a memory circuit 70 that stores information, a logic circuit 80 that controls overall function of the device, etc. The antenna circuit 20 for a typical RFID tag may be formed by a parallel resonant LC circuit, where L is inductance and C is capacitance.
When radio waves from a tag reader are encountered by the passive RFID tag 10, the antenna circuit 20 within the tag forms a magnetic field. The integrated circuit 30 draws power from the antenna, via the rectifier 40, energizing the circuits in the tag. The integrated circuit 30 uses that energy to transmit response codes by modulating the impedance the antenna circuit 20 presents to the interrogating field, thereby modulating the signal reflected back to the a tag reader antenna. Typical tag readers obtain a tag's ID information by requesting one bit at a time. The integrated circuit 30 on the tag processes signals from the tag reader to learn what bit is requested, and then sets backscatter modulation in order to respond with the requested bit. The integrated circuit 30 also provides control logic that allows tag readers to illuminate and communicate with multiple tags within its transmitted field. Without control logic, all illuminated tags would respond to queries from the tag reader simultaneously and cause interference with one another. Proper control logic and communication algorithms/protocols solve this multiple-tag collision problem. Unfortunately, the power requirements of the integrated circuit 30 largely drives the read range performance of passive RFID tags. Reading range is defined as the communication operating distance between the reader and tag. The reading range of a typical RFID tag may be less than ten meters, which is a significant limiting factor of passive RFID applications. Accordingly, improvements in passive RFID technologies are still needed.